1. Technical Field
The present invention relates to a head drive apparatus of an ink jet printer in which a plurality of nozzles jet minute ink drops of liquid ink of a plurality of colors and particles of the ink (ink dots) are formed on a print medium to draw pre-determined characters and images, and a head driving method.
2. Related Art
An ink jet printer as in the above generally accomplishes low-cost and high-quality color printed material easily. As such, it is widely used not only in offices but also by general users along with popularization of a personal computer and a digital camera.
Generally, in such an ink jet printer, a moving part called a carriage, for example, integrally comprising ink cartridges and print heads (also called as ink jet heads) moves back and forth on a print medium in a direction crossing a direction to convey the medium, and nozzles of the print head jet (eject) liquid ink drops to form minute ink dots on the print medium. In this manner, pre-determined characters or images are drawn on the print medium to create desired printed material. The carriage comprises ink cartridges for four colors including black (and yellow, magenta, cyan) and a print head for each of the colors, so that not only monochrome print but also full color print in combination of the respective colors can be easily performed (further, print in six colors including the colors, light cyan and light magenta, seven colors, and eight colors are practically implemented).
In the above type of ink jet printer for executing print by moving the ink jet heads on the carriage back and forth in a direction crossing a direction to convey a print medium in the above manner, the ink jet heads must be moved back and forth about ten times or more than tens of times to neatly print a whole page.
On the other hand, in an ink jet printer comprising ink jet heads (do not need to be integrated) of the same length as the width of a print medium but not comprising a carriage, the ink jet heads do not need to be moved in a width direction of the print medium so that one-pass printing is possible, enabling quick printing similar to a laser printer. An ink jet printer in the former scheme is generally called a “multi-pass (serial) ink jet printer”, while an ink jet printer in the latter scheme is generally called a “line head ink jet printer”.
In such an ink jet printer, drive pulses are used to drive an actuator to change the pressure in a pressurizing chamber, so that the pressure change causes the ink in the pressurizing chamber to be jetted as ink drops through a nozzle which is in communication with the pressurizing chamber. There are several types of actuators, and for example, in a piezoelectric ink jet printer, an application of a drive pulse to a piezoelectric element which is an actuator causes a vibrating plate in contact with a pressurizing chamber to be displaced, which changes the pressure in the pressurizing chamber, and ink drops are jetted.
A method for generating a drive pulse is illustrated in FIG. 2 of JP-A-2004-306434. That is, data is read out from a memory for storing drive signal data, a D/A converter converts the data into analog data, and a drive signal is supplied to an ink jet head through a current amplifier. A circuit of the current amplifier comprises transistors in push-pull connection, as shown in FIG. 3 of the document, in which a linear drive amplifies a drive signal.
In this type of an ink jet printer, for reduction of printing time, simplification of a driving circuit, reduction of the number of signal lines, and the like, a common drive pulse is applied to actuators of a plurality of nozzles. In other words, the same drive pulses are simultaneously supplied to a plurality of actuators, and in this case, the plurality of actuators is connected in parallel to one drive pulse. The connected actuators are selected in response to nozzles through which ink drops are jetted, that is, in response to print data. It has become apparent that when the number of actuators connected to one drive pulse changes as described above, the characteristic of the jet of ink drops changes in accordance with the number of the connected actuators. Therefore, in an ink jet printer described in the following JP-A-2000-238262, the number of actuators (or nozzles) which are actually driven is acquired, and then in accordance with the number, a drive pulse itself for jetting ink drops is changed and set. Specifically, a voltage gradient of a drive pulse of a voltage signal having a trapezoidal waveform signal or a peak voltage itself thereof is changed so as to stabilize the characteristics of jet of ink drops.